As for the reasons for using RGB instead of SML, I can now see why the sensor would use RGB since that is the "target" of a digital capture. How about removing RGB from all steps of the chain and capture, store and display SML? Is there any inherent advantage to RGB that means we might still be using it 30 years from now?

In theory any set of RGB's/SML may be used as long as one set can be derived from the other. For example all 6 in the following diagram may be used equivalently. However, in practise, some may be more advantageous, such as for reasons of physical realizability and gamut extent among others. For instance, the last two sets at the bottom left and right may be more useful for signal compression, noise suppression, color processing, etc. than the ones above, which for historic reasons are more popular in colorimetry.